Physiology, Muscle


Article Author:
Rachel Noto


Article Editor:
Mary Ann Edens


Editors In Chief:
Chaddie Doerr


Managing Editors:
Avais Raja
Orawan Chaigasame
Carrie Smith
Abdul Waheed
Khalid Alsayouri
Frank Smeeks
Kristina Soman-Faulkner
Trevor Nezwek
Radia Jamil
Patrick Le
Sobhan Daneshfar
Anoosh Zafar Gondal
Saad Nazir
William Gossman
Pritesh Sheth
Hassam Zulfiqar
Navid Mahabadi
Steve Bhimji
John Shell
Matthew Varacallo
Heba Mahdy
Ahmad Malik
Mark Pellegrini
James Hughes
Beata Beatty
Nazia Sadiq
Hajira Basit
Phillip Hynes
Tehmina Warsi


Updated:
10/27/2018 12:31:44 PM

Introduction

There are 3 major muscle types in the human body: skeletal, cardiac, and smooth muscle. Each muscle type has unique cellular components, physiology, specific functions, and pathology. Skeletal muscle is an organ that controls movement and posture. Cardiac muscle encompasses the heart, which keeps the human body alive. Smooth muscle is located throughout the gastrointestinal, reproductive, urinary, vascular, and respiratory systems.

Cellular

Skeletal muscle constitutes approximately 40% of the total human body weight. It is comprised of many individual fibers that are bundled together into a muscle spindle; this is what gives the skeletal muscle a striated appearance. A single muscle fiber is composed mostly of actin and myosin fibers covered by a cell membrane (sarcolemma). These fibers are the functional unit of the organ, leading to contraction and relaxation. There are 2 major classifications of skeletal muscle; these include Type I (slow oxidative) and Type II (fast twitch). The vast diversity in the makeup of skeletal muscle leads to variations in speed and length of contractions in different muscle groups depending on specific function.[1]

Cardiac muscle or myocardium is an involuntary, striated muscle that encloses the chambers of the heart. It is comprised of individual cardiomyocytes, which are similar in structure to skeletal muscle. Each cardiomyocyte contains cytoskeletal and contractile elements, all of which are connected through intercalated discs. These are highly adherent complexes, which allow the cardiac muscle cells to receive rapid electrical transmission and contract as a single unit.[2] Cardiac muscle also contains specialized cardiac pacemaker cells that lie within the myocardium. These cells allow for cardiac tissue to depolarize without external stimuli intrinsically.[3]

The cells of smooth muscle are also composed of actin and myosin fibers, but they are arranged in sheets rather than spindles to give the muscle a smooth appearance. These cells are contained in the walls of many organs such as the lungs, gastrointestinal tract, reproductive organs, blood vessels, and even in the skin.[4]

Function

Muscle in the human body, whether it is skeletal, cardiac or smooth, functions to create force and movement. Muscles of the skeleton support the bones to maintain posture as well as control voluntary movement. Skeletal muscle also contributes to energy metabolism and storage. Cardiac muscle propels blood and leads to proper oxygenation and maintenance of each cell that comprises the human body. Smooth muscle is located throughout the body and uses contractile force to shorten and propel various contents across the lumen of the multiple organ systems in which it is involved.

Mechanism

Action potentials from nerve fibers of the central nervous system depolarize muscle down the length of the sarcolemma to the innermost fibers through a transverse tubule (T tubule) system. The action potential responds with a dihydropyridine receptor on the T tubule; this acts as a voltage sensor allowing for calcium to be released. Calcium then activates ryanodine receptors in the sarcoplasmic reticulum to release even more calcium. Higher quantities of calcium can then bind to the protein troponin located on the actin fibers. The calcium-troponin complex displaces the protein tropomyosin from the active site of the actin filament and allows for myosin binding and muscle contraction. ATP is needed to detach myosin from actin and allow for relaxation.[1]

Similarly to skeletal muscle, cardiac muscle is triggered by calcium binding to troponin in the actin filaments of the cardiomyocyte. This then removes tropomyosin and allows for the binding of myosin to actin and eventual contraction. The significant difference between cardiac and skeletal muscle is in cardiomyocytes automaticity. Specialized cardiac pacemaker cells located in the sinoatrial (SA) node are responsible for creating cardiac muscle contraction. These act to trigger action potentials which allow for sodium and potassium influx as well as calcium release from the sarcoplasmic reticulum. The cardiac muscle can then contract as a single unit.[5]

Smooth muscle contraction is not under voluntary control and is done so through autonomic regulation of a calcium-calmodulin interaction. Contraction begins through a change in action potential or activation of mechanical stretch receptors in the plasma membrane. Intracellular calcium is increased and combines with the protein calmodulin. It is this complex which activates the myosin light chain (MLC) kinase to phosphorylate and form cross-bridges between myosin and actin, leading to contraction. Some smooth muscle maintains tone, which is caused by a constant phosphorylation level in the absence of external potentials. A decrease in intracellular calcium levels induces relaxation.[4]

Clinical Significance

Muscular dystrophy is a progressive genetic myopathy, which leads to degeneration of normal anatomy and physiology of skeletal muscle cells. The complete or partial absence of the dystrophin protein is the mechanism of both Becker and Duchenne muscular dystrophy. Dystrophin is a protein that is associated with the filaments of skeletal muscle. Dystrophin provides structure and support to the sarcolemma of the monofilament. Lack of dystrophin protein leads to damage in the supporting sarcolemma, weakness and eventual atrophy of healthy muscle fibers. Duchenne muscular dystrophy affects up to 1 in 3600 boys, which makes it the highest incidence upon the muscular dystrophies. Many with Duchenne have a low life expectancy because there is currently no treatment available. Management of these disorders is purely supportive. The most common cause of death in these individuals is cardiorespiratory failure.[6]

 Sarcopenia is the loss of muscle mass and atrophy that is associated with aging. It has been attributed to a reduction of muscle size as well as a reduction in satellite cells, mitochondrial numbers, and elasticity. Sarcopenia is seen in increasing numbers with advancing age but is not universal. Sarcopenia varies in degree of physical activity, gender, and race. It can attribute to loss of muscle power and immobility issues such as falls, commonly seen in aging populations.[1]

Smooth muscle cells can be found lining the entirety of the human vascular system. They have been shown to exhibit plasticity in response to vascular injury. It is this plasticity that has been implicated in the disease process of atherosclerosis. Mature smooth muscle cells are involved in contraction and tone in the vascular system. Cholesterol load has been shown to increase stress on endothelial cells, leading to vascular injury. This damage changes the vascular smooth muscle from the inactive contractile state to the pro-inflammatory response state. Smooth muscle cell proliferation and remodeling then results, this leads to the fibrous capsule formation seen in atherosclerosis.[7]

Hypertrophic obstructive cardiomyopathy (HOCM) is an autosomal dominant disorder caused by genetic variants that code for a portion of the contractile element of the cardiomyocyte. These mutations allow for heightened myofilament calcium sensitivity, thickening of the interventricular septum and eventual obstruction of blood flow. Although commonly asymptomatic, symptoms of obstruction can result in chest pain during exertion, tachycardia with shortness of breath, syncope, and sudden cardiac death. HOCM is the most commonly inherited cardiac disorder with a prevalence of 1 in 500. It is the leading cause of sudden death in young individuals and currently has no cure.[8]


Interested in Participating?

We are looking for contributors to author, edit, and peer review our vast library of review articles and multiple choice questions. In as little as 2-3 hours you can make a significant contribution to your specialty. In return for a small amount of your time, you will receive free access to all content and you will be published as an author or editor in eBooks, apps, online CME/CE activities, and an online Learning Management System for students, teachers, and program directors that allows access to review materials in over 500 specialties.

Improve Content - Become an Author or Editor

This is an academic project designed to provide inexpensive peer-reviewed Apps, eBooks, and very soon an online CME/CE system to help students identify weaknesses and improve knowledge. We would like you to consider being an author or editor. Please click here to learn more. Thank you for you for your interest, the StatPearls Publishing Editorial Team.

Physiology, Muscle - Questions

Take a quiz of the questions on this article.

Take Quiz
Which of the following is the main determinant that increases blood flow within exercising skeletal muscle?



Click Your Answer Below


Would you like to access teaching points and more information on this topic?

Improve Content - Become an Author or Editor and get free access to the entire database, free eBooks, as well as free CME/CE as it becomes available. If interested, please click on "Sign Up" to register.

Purchase- Want immediate access to questions, answers, and teaching points? They can be purchased above at Apps and eBooks.


Sign Up
A male lifts a 250 pound barbell, holds it up for 5 seconds, and his arms give way and he drops it. Sudden muscle relaxation is caused by which receptors?



Click Your Answer Below


Would you like to access teaching points and more information on this topic?

Improve Content - Become an Author or Editor and get free access to the entire database, free eBooks, as well as free CME/CE as it becomes available. If interested, please click on "Sign Up" to register.

Purchase- Want immediate access to questions, answers, and teaching points? They can be purchased above at Apps and eBooks.


Sign Up
Radiolabeled ATP is injected into an isolated muscle, which is in turn stimulated and contracts for 10 seconds. The ATP will be shown bound to which structure?



Click Your Answer Below


Would you like to access teaching points and more information on this topic?

Improve Content - Become an Author or Editor and get free access to the entire database, free eBooks, as well as free CME/CE as it becomes available. If interested, please click on "Sign Up" to register.

Purchase- Want immediate access to questions, answers, and teaching points? They can be purchased above at Apps and eBooks.


Sign Up
A patient is running on a treadmill. Skeletal muscles will show what change during exercise?



Click Your Answer Below


Would you like to access teaching points and more information on this topic?

Improve Content - Become an Author or Editor and get free access to the entire database, free eBooks, as well as free CME/CE as it becomes available. If interested, please click on "Sign Up" to register.

Purchase- Want immediate access to questions, answers, and teaching points? They can be purchased above at Apps and eBooks.


Sign Up
A 17-year-old man is lifting a 200 pound barbell and holds it up for 8 seconds and his arms give way and he drops it. Sudden muscle relaxation is caused by which receptors?



Click Your Answer Below


Would you like to access teaching points and more information on this topic?

Improve Content - Become an Author or Editor and get free access to the entire database, free eBooks, as well as free CME/CE as it becomes available. If interested, please click on "Sign Up" to register.

Purchase- Want immediate access to questions, answers, and teaching points? They can be purchased above at Apps and eBooks.


Sign Up
Select the type of muscle that produces the greatest range of motion.



Click Your Answer Below


Would you like to access teaching points and more information on this topic?

Improve Content - Become an Author or Editor and get free access to the entire database, free eBooks, as well as free CME/CE as it becomes available. If interested, please click on "Sign Up" to register.

Purchase- Want immediate access to questions, answers, and teaching points? They can be purchased above at Apps and eBooks.


Sign Up
A bodybuilder is doing arm curls in the gym. Which of the following is the mechanism of action for shortening of the biceps muscle?



Click Your Answer Below


Would you like to access teaching points and more information on this topic?

Improve Content - Become an Author or Editor and get free access to the entire database, free eBooks, as well as free CME/CE as it becomes available. If interested, please click on "Sign Up" to register.

Purchase- Want immediate access to questions, answers, and teaching points? They can be purchased above at Apps and eBooks.


Sign Up
Which neurotransmitter is needed for the depolarization of skeletal muscle?



Click Your Answer Below


Would you like to access teaching points and more information on this topic?

Improve Content - Become an Author or Editor and get free access to the entire database, free eBooks, as well as free CME/CE as it becomes available. If interested, please click on "Sign Up" to register.

Purchase- Want immediate access to questions, answers, and teaching points? They can be purchased above at Apps and eBooks.


Sign Up
How is an isometric twitch similar to an isotonic twitch?



Click Your Answer Below


Would you like to access teaching points and more information on this topic?

Improve Content - Become an Author or Editor and get free access to the entire database, free eBooks, as well as free CME/CE as it becomes available. If interested, please click on "Sign Up" to register.

Purchase- Want immediate access to questions, answers, and teaching points? They can be purchased above at Apps and eBooks.


Sign Up
During muscle contraction, an action potential reaches the axon terminal and causes a subsequent release of calcium ions and causes acetylcholine release from synaptic vesicles. Which of the following occurs next?



Click Your Answer Below


Would you like to access teaching points and more information on this topic?

Improve Content - Become an Author or Editor and get free access to the entire database, free eBooks, as well as free CME/CE as it becomes available. If interested, please click on "Sign Up" to register.

Purchase- Want immediate access to questions, answers, and teaching points? They can be purchased above at Apps and eBooks.


Sign Up
Which of the following is a true statement regarding the differences in contraction of smooth and skeletal muscle?



Click Your Answer Below


Would you like to access teaching points and more information on this topic?

Improve Content - Become an Author or Editor and get free access to the entire database, free eBooks, as well as free CME/CE as it becomes available. If interested, please click on "Sign Up" to register.

Purchase- Want immediate access to questions, answers, and teaching points? They can be purchased above at Apps and eBooks.


Sign Up

Physiology, Muscle - References

References

Frontera WR,Ochala J, Skeletal muscle: a brief review of structure and function. Calcified tissue international. 2015 Mar     [PubMed]
Roth GM,Bader DM,Pfaltzgraff ER, Isolation and physiological analysis of mouse cardiomyocytes. Journal of visualized experiments : JoVE. 2014 Sep 7     [PubMed]
Burkhard S,van Eif V,Garric L,Christoffels VM,Bakkers J, On the Evolution of the Cardiac Pacemaker. Journal of cardiovascular development and disease. 2017 Apr 27     [PubMed]
Sevrieva I,Knowles AC,Kampourakis T,Sun YB, Regulatory domain of troponin moves dynamically during activation of cardiac muscle. Journal of molecular and cellular cardiology. 2014 Oct     [PubMed]
Webb RC, Smooth muscle contraction and relaxation. Advances in physiology education. 2003 Dec     [PubMed]
Shieh PB, Muscular dystrophies and other genetic myopathies. Neurologic clinics. 2013 Nov     [PubMed]
Chistiakov DA,Orekhov AN,Bobryshev YV, Vascular smooth muscle cell in atherosclerosis. Acta physiologica (Oxford, England). 2015 May     [PubMed]
Robinson P,Liu X,Sparrow A,Patel S,Zhang YH,Casadei B,Watkins H,Redwood C, Hypertrophic cardiomyopathy mutations increase myofilament Ca{sup}2 {/sup} buffering, alter intracellular Ca{sup}2 {/sup} handling, and stimulate Ca{sup}2 {/sup}-dependent signaling. The Journal of biological chemistry. 2018 Jul 6     [PubMed]

Disclaimer

The intent of StatPearls is to provide practice questions and explanations to assist you in identifying and resolving knowledge deficits. These questions and explanations are not intended to be a source of the knowledge base of all of medicine, nor is it intended to be a board or certification review of Nurse-Physiology. The authors or editors do not warrant the information is complete or accurate. The reader is encouraged to verify each answer and explanation in several references. All drug indications and dosages should be verified before administration.

StatPearls offers the most comprehensive database of free multiple-choice questions with explanations and short review chapters ever developed. This system helps physicians, medical students, dentists, nurses, pharmacists, and allied health professionals identify education deficits and learn new concepts. StatPearls is not a board or certification review system for Nurse-Physiology, it is a learning system that you can use to help improve your knowledge base of medicine for life-long learning. StatPearls will help you identify your weaknesses so that when you are ready to study for a board or certification exam in Nurse-Physiology, you will already be prepared.

Our content is updated continuously through a multi-step peer review process that will help you be prepared and review for a thorough knowledge of Nurse-Physiology. When it is time for the Nurse-Physiology board and certification exam, you will already be ready. Besides online study quizzes, we also publish our peer-reviewed content in eBooks and mobile Apps. We also offer inexpensive CME/CE, so our content can be used to attain education credits while you study Nurse-Physiology.